Vfan jumper has 3 pins. One is always +5v from the 5v regulator on the duet (or possibly external 5v if you have that jumper set). One is Vin from your psu so 12 or 24v usually. The jumper connects the fan positive bus which runs to all the fan headers both pwm and always-on, to either 5v or Vin depending on the jumper connection.

So chose one.

However you can be clever with it. I have some 5v fans and some 24v fans. The 5v fans positive comes off the +5v pin of the vfan header and are grounded via the fan header GND of your choice. The pwm is achieved through the gnd. The rest of my fans plug into the fan headers directly and my vfan jumper is set to Vin (I have a 24v psu).

You can also have 12v fans on a 24v system by connecting 12v to the central pin of vfan header.